I assume you are referring to lair actions or similar effects. Initiative count 20 refers to where they occur in the initiative order. So after all combatants roll initiative you get the order of fighters. Whenever the initiative would cross 20, that effect occurs. So for example let's say there are three combatants who roll initiative 25, 18, and 10. On a round when the effect might take place the combatant who had initiative 25 would go first in the round, then the effect would happen as we pass initiative count 20, then the combatant with 18, then the one with 10